Cancer causes 1 of every 4 deaths in the US. The development of fundamentally new, clinically useful anticancer drugs therefore constitutes a national health and research imperative. Leinamycin (LNM), iso-migrastatin (iso-MGS), migrastatin (MGS), and lactimidomycin (LTM) are promising anticancer drug leads with unprecedented modes of action. A great challenge is to develop ways to prepare these complex natural products and their structural analogs for mechanistic studies and clinical development. We propose in this Competitive Renewal application (i) to continue to study LNM, iso-MGS, and LTM biosynthesis to discover novel chemistry and enzymology and (ii) to apply combinatorial biosynthesis methods to the LNM, iso-MGS, and LTM biosynthetic machinery for production of novel anticancer drugs. Our hypotheses are (i) the LNM, iso-MGS, and LTM AT-less type I PKSs represent a novel PKS architecture, the studies of which will reveal new insights into the molecular mechanism of PKS catalysis, (ii) AT-less type I PKSs provide new opportunities for PKS engineering, methods and strategies for expanding polyketide structural diversity by combinatorial biosynthesis, (iii) several other aspects in LNM, iso-MGS, and LTM biosynthesis are unprecedented, the characterization of which will uncover new chemistry and enzymology, and (iv) LNM, iso-MGS, MGS, and LTM are excellent anticancer leads with novel modes of action, and these natural products and their structural analogs could be realistically developed into new anticancer drugs. The specific aims for this grant period are: (i) mechanistic and structural characterization of the LNM, iso-MGS, and LTM AT-less type I PKSs as models to investigate how AT-less type I PKS interacts with its cognate acyltransferase (AT) to constitute a functional PKS megasynthase for polyketide biosynthesis; (ii) mechanistic and structural characterization of novel enzymes for LNM, iso-MGS, and LTM biosynthesis; and (iii) rational engineering of the LNM, iso-MGS, and LTM pathways for metabolite overproduction and novel analogs. The outcomes of these studies include the discovery and development of novel anticancer drug leads and potentially clinically useful anticancer drugs. The long-term goal of our research is to understand at a molecular level how microorganisms synthesize complex natural products and to develop and apply combinatorial biosynthesis methods to natural products for anticancer drug discovery and development.